1. Field of the Invention
The present invention relates to laser printing and, in particular, discloses a high volume colour laser printing system.
2. Description of the Prior Art
Colour laser copiers and printing systems are known in the art with one well known example being the CANON (Registered Trade Mark) Colour Laser Copier from Canon KK of Japan.
Such copiers and corresponding printers have the potential to become standard engines for colour desktop publishing as well as for many other areas of printed image generation. However, there are several problems which should be overcome in order to fully realise the potential of such equipment.
Specifically, one of these problems is printer speed. Colour laser copiers currently in use have a print speed of five A4 pages per minute (PPM) or three A3 PPM. Such speeds limit the practicality of producing large print runs on the copier. Generally, off-set printing is used for volumes in excess of about 100 pages, however off-set printing involves attendant setup costs and delays.
Turning now to FIG. 1, a prior art colour laser copier is shown in schematic block diagram. A scanner 1 is provided to scan the document to be reproduced and outputs scanned digital data via a bus 3 to a digital signal processing unit 2. This unit 2 outputs to a digital to analogue converter (DAC) and pulse width modulator (PWM) 4 via a bus 5. The busses 3 and 5 carry the video data in its component colours as red, green and blue (RGB) and magenta, cyan, yellow, and black (MCYK) respectively. Generally, in the former 24 bits in parallel are used and in the latter 4.times.8 serial bits of information are used.
The unit 4 supplies a laser scanner 6 which sensitises the drum (not illustrated) of an electrophotographic unit in known fashion.
The system used by the Canon laser copier utilises four component colours M, C, Y, and K and uses a 13.35 MBytes per second data rate. A control microprocessor 8 is provided to provide overall process control of the copier.
The copier of FIG. 1 can be readily divided into two parts comprising an source unit 10 and a colour laser printer (CLP) engine 20. A block diagram representation of such a system is shown in FIG. 2 where the image source 10 and the CLP engine 20 are clearly shown as individual devices connected by a number of electrical connections. The CLP engine 20 accordingly represents the printing part of the colour laser copier and such an engine, when divided from a Canon laser copier accepts 8 bit MCYK video data running at 13.35 MBytes per second. The source unit 10 can be a scanner as in the scanner of FIG. 1, or can also be a computer graphic system, such as that disclosed in Australian Patent Application Nos. PK1785 and PK3418 to the present Applicant, from which images can be produced electronically and printed using the CLP engine 20. The source unit 10 includes an image source 11 which outputs data to a line first-in-first-out (FIFO) shift register 12 which is able to hold one line of data for printing in the CLP engine 20. Generally, this line is 4632 bytes and in practice a 5K byte FIFO 12 is used. The source unit 10 also includes a balanced transmitter 13 which outputs data from the line FIFO 12 to the CLP engine 20. A synchronisation unit 6 is provided to synchronise the transfer of data from the source unit 10 to the CLP 20. An RS232 unit 15 is provided also for the communication of control signals between the unit 10 and the engine 20.
The CLP engine 20 includes a balanced receiver 14 for receiving colour component video data from the source unit 10. The received data is stored in a line FIFO register 21 which in turn outputs to a signal conditioner and pulse width modulator 4 in a manner similar to that of FIG. 1. The conditioner and modulator 4 in turn outputs to the laser beam printer 7. An RS232 communication port 16 is also provided to receive command data from the source unit 10.
Accordingly, the CLP engine 20 represents that portion of a colour laser printer stripped of all functions not necessary for actually printing the duplicate document. Accordingly, such an engine does not require a frame store, a computer except a microcontroller, a scanner, complicated graphic software such a Postscript and other such equipment and features which are utilised in a complete laser colour copier.
Accordingly, the cost of such an engine is substantially reduced when compared to the cost of laser copier. It is envisaged that the engine can retail for about US$5,000 in 1993.
High volume printing can be achieved in a number of ways and one method is by printing at a higher speed which involves the construction of a faster printer. However, in order to obtain an n-fold increase in print speed, all component parts of the printer are required to operate n-times faster. Such a printer is shown in block diagram form in FIG. 3. FIG. 3 shows a high speed system which includes a high speed scanner 30 which outputs MCYK data via a bus 31 to a high speed printer 32. An optional graphics system 33 can also be provided to convert RGB data from the scanner 30 into MCYK data for the printer 32.
It is apparent to those skilled in the art that in such a system, every component must operate at a proportionally faster rate. For example, to obtain a 10-fold improvement in printing speed, the following is required:
The MCYK data rate must increased to 133.5 MBytes per second;
The digital signal processing in the print engine must be made to work ten times faster;
The drum(s) must rotate ten times faster;
The paper transport mechanism(s) must operate ten times faster;
The laser switching speed must be increased by a factor of ten;
The laser power must be increased so as to charge the selenium drum (used in the Canon laser copier) to the same level;
The polygon mirror must rotate ten times faster, whilst maintaining the same level of accuracy;
The toner fixing process must operate ten times faster;
The toner transport mechanism must operate ten times faster;
A scanner capable of operating at a speed ten times faster must be built; and
All the computer graphics equipment which connects to and drives the printer must also operate ten times faster.
Those skilled in the art will appreciate that the combined set of problems outlined above is most severe. Also, a substantial development effort would be required to obtain even a modest speed improvement, and accordingly, such an option is not readily available. Unit costs are envisaged as being in excess of US$60,000.
Existing colour laser print engines, such as that used in the Canon colour laser copier, use a four stage print process in which each sheet of paper undergoes one pass around the print drum for each of the magenta, cyan, yellow and black print stages. It is therefore possible to make a printer which operates approximately four times faster by including separate print drums for each colour. Therefore, while the first page is being printed with black, the second is printed with yellow, the third with cyan and the fourth with magenta. Such a printer is shown in block diagram form in FIG. 4. In such a printer, the time taken to print one page is not reduced but however, the total page throughput is increased by a factor of four where multiple copies are required. With reference to FIG. 4, a parallel output scanner outputs MCYK data on each of four corresponding busses 41. The busses 41 supply a four drum printer 42 with each drum adapted for printing the specific colour required. As in the system of FIG. 3, an optional graphic system 44 can be provided to convert RGB data into MCYK data.
The development of a printer such as that shown in FIG. 4, includes the following:
Parallel MCYK data ports must be added;
Four digital signal processing units within the print engine must be included;
Four lasers, polygon mirrors, selenium drums, tone units, developer units and paper transport mechanisms must be included;
Large FIFO's must be included to accommodate for the difference between the start times of the MCYK printing frames caused by mechanical tolerances within the print mechanisms;
The paper transport mechanism must be redesigned to pass the paper from one drum to the next;
A scanner with parallel MCYK outputs must be built; and
All computer graphics equipment which connects to the printer must have parallel MCYK data outputs.
Those skilled in the art will realise that, like FIG. 3, substantial development effort is required to implement the system of FIG. 4. Whilst the development is relatively straight forward, integration of known technology into such a device is not trivial. Also, in view of the extended capacity of the printer of FIG. 4, individual item cost would be substantial. Unit costs are envisaged as being about US$40,000.
The foregoing discussion identifies a number of ways by which high volume printing can be achieved. However, each of these ways involve either substantial cost, or, substantial redevelopment of existing technology.